Fin replacement in a field-effect transistor

ABSTRACT

In a method for fabricating a field-effect transistor (FET) structure, forming a fin on a semiconductor substrate. The method further includes forming a gate on a portion of the fin and the semiconductor substrate. The method further includes epitaxially growing a semiconductor material on the fin. The method further includes depositing oxide covering the fin and the epitaxially grown semiconductor material. The method further includes recessing the deposited oxide and the epitaxially grown semiconductor material to expose a top portion of the fin. The method further includes removing the fin. In another embodiment, the method further includes epitaxially growing another fin in a respective trench formed by removing the first set of fins.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of semiconductordevices, and more particularly to replacing fins in a field-effecttransistor.

Field-effect transistors (FETs) can be semiconductor devices fabricatedon a bulk semiconductor substrate or on a silicon-on-insulator (SOI)substrate. FET devices generally consist of a source, a drain, a gate,and a channel between the source and drain. The gate is separated fromthe channel by a thin insulating layer, typically of silicon oxide,called the gate oxide. A voltage applied between the source and the gateinduces an electric field that modulates the conductivity of the channelbetween the source and the drain thereby controlling the current flowbetween the source and the drain. Current integrated circuit designs usecomplementary metal-oxide-semiconductor (CMOS) technology that usecomplementary and symmetrical pairs of p-type and n-type metal oxidesemiconductor field-effect transistors (MOSFETs) for logic functions.

Silicon-germanium (SiGe) is a general term for the alloy Si_(1-x)Ge_(x),which consists of any molar ratio of silicon (Si) and germanium (Ge).SiGe can be used as a semiconductor material in integrated circuits as astrain-inducing layer for CMOS transistors. SiGe is manufactured onsilicon wafers using conventional silicon processing toolsets.

SUMMARY

One aspect of the present invention discloses a method for fabricationof a field-effect transistor (FET) structure. The method includesforming a fin on a semiconductor substrate. The method further includesforming a gate on a portion of the fin and the semiconductor substrate.The method further includes epitaxially growing a semiconductor materialon the fin. The method further includes depositing oxide covering thefirst set of fins and the epitaxially grown semiconductor material. Themethod further includes recessing the deposited oxide and theepitaxially grown semiconductor material to expose a top portion of thefin. The method further includes removing the fin. In anotherembodiment, the method further includes epitaxially growing another finin a respective trench formed by removing the fin.

Another aspect of the present invention discloses a field-effecttransistor (FET) structure. The FET structure comprises a fin formed ona semiconductor substrate. The FET structure further comprises a gateformed on a portion of the fin and the semiconductor substrate, whereinthe portions of the fin under the gate is a different material than theportions of the fin that are not under the gate. The FET structurefurther comprises epitaxially grown semiconductor material on the fin.The FET structure further comprises an oxide covering at least a portionof the epitaxially grown semiconductor material and the fin.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the disclosure solely thereto, will best beappreciated in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a flowchart illustrating an exemplary method offabricating a field-effect transistor (FET), in accordance withembodiments of the present invention.

FIG. 2A depicts a top-down view of a FET with a formation of fins andgates on a substrate, in accordance with embodiments of the presentinvention. FIG. 2B depicts a cross-sectional view of the formation offins and gates of FIG. 2A, in accordance with embodiments of the presentinvention.

FIG. 3A depicts a top-down view of epitaxial growth on the fins in thesource and drain region of the FET of FIG. 2A, in accordance withembodiments of the present invention. FIG. 3B depicts a cross-sectionalview of the FET structure of FIG. 3A, in accordance with embodiments ofthe present invention.

FIG. 4A depicts a cross-sectional view of oxide deposited into the FETstructure of FIG. 3B, in accordance with embodiments of the presentinvention. FIG. 4B depicts a cross-sectional view of the oxide andepitaxy of FIG. 4A recessed to expose the fins, in accordance withembodiments of the present invention.

FIG. 5A depicts a cross-sectional view of the exposed fins of FIG. 4Brecessed and removed, in accordance with embodiments of the presentinvention. FIG. 5B depicts fins regrown in the recessed portions of FIG.5A, in accordance with embodiments of the present invention.

FIG. 6A depicts a cross-sectional view of epitaxial growth on the top ofthe regrown fins of FIG. 5B, in accordance with embodiments of thepresent invention. FIG. 6B depicts a cross-sectional view of a recess tothe oxide of FIG. 6A, in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely illustrative of the claimed structures and methods that maybe embodied in various forms. In addition, each of the examples given inconnection with the various embodiments is intended to be illustrativeand not restrictive. Further, the Figures are not necessarily to scale,some features may be exaggerated to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the methods and structures of the present disclosure.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc. indicate that the embodiment described mayinclude a particular feature, structure, or characteristic, but everyembodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on,” “positioned on,” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element anda second element are connected without any intermediary conducting,insulating, or semiconductor layers at the interface of the twoelements.

Some embodiments of the present invention recognize that it can bedesirable to utilize silicon-germanium (SiGe) in source/drain epitaxy toincrease strain and enhance performance of a field-effect transistor(FET) device. Junction formation in FinFET is accomplished utilizingout-diffusion from source/drain merge epitaxy into a channel regionunder a spacer. High dopant levels are utilized for contact formation,and very high dopant levels are desirable to form good contact andachieve low contact resistance. High dopant level close to the gate canlead to high diffusion of dopant under the spacer during extensionformation anneal and are capable of shorting the device. Embodiments ofthe present invention recognize a trade-off between good junctionformation (without shorting) and high doping levels for good contactformation (low contact resistance). Additional embodiments of thepresent invention recognize that anneals can relax the strain in theSiGe and eliminating the need for anneal is further beneficial.

Embodiments of the present invention generally provide a method toselectively remove un-doped fins in the source/drain region after anunmerged epitaxy has been formed around the un-doped fins. New fins canthen be regrown utilizing a different material. For example, new finscan be comprised of SiGe, Boron doped Silicon, Boron doped SiGe, higherpercentage SiGe, or other types of materials. Replacing the fin withdoped material can reduce (or eliminate) the need for diffusionprocesses and additionally can provide the benefit of additionalstressor material. In additional embodiments, after fin removal andbefore fin regrowth, the method can etch the fin under the space, whichcan move the junction and stressor material even closer to under thechannel region.

The present invention will now be described in detail with reference tothe Figures. FIG. 1 is a functional block diagram illustratingfabrication process 100, an exemplary method for fabricating a FET, inaccordance with one embodiment of the present invention.

In step 102, fabrication process 100 forms a set of fins and gates on asubstrate. In one embodiment, fabrication process 100 forms a set offins on a substrate and a set of corresponding gates on the formedstructure of fins and the substrate. In various embodiments, thesubstrate can be a semiconductor on insulator (SOI) substrate, which caninclude a buried oxide (BOX) layer. In an example, fabrication process100 forms FET 200 (depicted in FIG. 2A), which includes substrate 202,fins 203, and gates 204. FIG. 2A depicts a top-down view of FET 200.Fabrication process 100 forms the set of fins 203 and gates 204 onsubstrate 202 utilizing conventional semiconductor fabricationtechniques. In another embodiment, FET 200 can include a differentnumber of fins in fins 203 and a different number of gates in gates 204(e.g., more or less fins and gates). In an example embodiment, fins 203are un-doped Si fins.

Substrate 202 may be composed of a silicon containing material. Siliconcontaining materials include, but are not limited to, Si, single crystalSi, polycrystalline Si, SiGe, single crystal silicon germanium,polycrystalline silicon germanium, or silicon doped with carbon,amorphous Si, and combinations and multi-layers thereof. Substrate 202may also be composed of other semiconductor materials, such as germaniumand compound semiconductor substrates, such as type III/V semiconductorsubstrates, e.g., GaAs. Although substrate 202 is depicted as a siliconon insulator substrate (SOI) or semiconductor on insulator substrate,bulk semiconductor substrates arrangements are also suitable forsubstrate 202. In additional embodiments, substrate 202 contains adielectric coating over the bulk semiconductor to isolate thesource/drain/gate metals, keeping the source/drain/gate metals fromshorting. The dielectric coating can be SiO₂ (thermal, plasma-enhancedchemical vapor deposition (PECVD), (low temperature oxide (LTO)), Al₂O₃or HfO₂ (e.g., atomic layer deposition (ALD) deposited), Si₃N₄ (siliconnitride), etc. In another embodiment, substrate 202 is a sapphiresubstrate (e.g., Al₂O₃ bulk).

In another example, FET 210 (FIG. 2B) depicts a cross-sectional view ofFET 200 from the perspective of cross-section line 205. FET 210 depictsa cross-sectional view of the array of fins 203 and gate 214, which isone of the gates of gates 204. FET 210 includes substrate 211 and BOX212. In various embodiments, BOX 212 can be silicon oxide (SiO₂) thatacts to insulate the formation of fins and gates (e.g., fins 203 andgates 204) from substrate 211. BOX 212 can be formed by thermallyoxidizing the exposed surface of substrate 211 or may be deposited ontosubstrate 211 using, for example, chemical vapor deposition (CVD) oratomic layer deposition (ALD). Fabrication process 100 can then bondfins 203 and gates 204 to BOX 212.

In step 104, fabrication process 100 grows SiGe on the fins. In oneembodiment, fabrication process 100 epitaxially grows SiGe on the fins(formed in step 102) in the source and drain regions of the FET. Invarious embodiments, the source and drain regions are the portions ofthe FET that are not covered by a gate (e.g., a gate formed in step102). Fabrication process 100 grows SiGe on the fins to form an unmergedepitaxy on the fins, which means that the epitaxial growth on one findoes not touch (or merge with) the epitaxial growth on another fin. Inexample embodiments, fabrication process 100 epitaxially grows highlydoped SiGe (e.g., 35% SiGe) on exposed surfaces of the fins (formed instep 102). In another embodiment, fabrication process 100 epitaxiallygrows SiGe on the fins, forming an unmerged diamond-shaped epitaxy inthe source and drain region of the FET. In other embodiments,fabrication process 100 can grow semiconductor materials, other thanSiGe or a different concentration of SiGe, etc. Examples of variousepitaxial growth process apparatuses that may be suitable for useperforming the epitaxy may include, for example, rapid thermal chemicalvapor deposition (RTCVD), low-energy plasma deposition (LEPD),ultra-high vacuum chemical vapor deposition (UHVCVD), atmosphericpressure chemical vapor deposition (APCVD), and molecular beam epitaxy(MBE).

In an example, fabrication process 100 epitaxially grows SiGe on fins203 in the source and drain regions of FET 200 and FET 210, which formsFET 300 (depicted in FIG. 3A) and FET 310 (depicted in FIG. 3B). FIG. 3Adepicts a top-down view of FET 300, which includes SiGe epitaxy 301.SiGe epitaxy 301 is grown on fins 203 (of FET 200 and FET 210) and is anunmerged epitaxy. In an example embodiment, SiGe epitaxy 301 is a highlydoped SiGe (e.g., 35% SiGe), which is grown as a diamond-shaped epitaxy.

In another example, FET 310 (FIG. 3B) depicts a cross-sectional view ofFET 300 from the perspective of cross-section line 305. FET 310 depictsa cross-sectional view of the diamond-shaped epitaxial growth of SiGeepitaxy 311 on fins 203. The diamond-shaped epitaxy of SiGe epitaxy 311is unmerged (e.g., the diamond-shaped epitaxial growth on fins 203 donot touch). In an example embodiment, SiGe epitaxy 311 is a highly dopedSiGe (e.g., 35% SiGe). In various embodiments, the diamond shapeobserved in the unmerged source-drain regions of FET 310 (and FET 300)may be a result of different growth rates during the epitaxialdeposition process inherent to each crystallographic orientation planeof the single-crystal material forming SiGe epitaxy 311 (and SiGeepitaxy 301). In other embodiments, SiGe epitaxy 311 (and SiGe epitaxy301) may have a shape other than the diamond shape depicted in FIG. 3B.

In step 106, fabrication process 100 deposits oxide. In one embodiment,fabrication process 100 deposits an oxide that fills in the gaps of theunmerged SiGe epitaxy (from step 104) and covers the SiGe epitaxy on thefins (e.g., but does not cover the gates). In example embodiments,fabrication process 100 can deposit a flowable oxide, silicon dioxide(SiO₂), or another material that is capable of filling the gaps in theSiGe epitaxy.

In an example, fabrication process 100 deposits flowable oxide 401 onFET 400 (depicted in FIG. 4A). Fabrication process 100 deposits flowableoxide 401 filling in and covering SiGe epitaxy 311 on fins 203. Forexample, flowable oxide 401 fills in the gaps that are the result of theunmerged epitaxy on fins 203 (from step 104). In example embodiments,flowable oxide 401 can be any type of flowable oxide that is capable offilling in the gaps of the unmerged epitaxy (e.g., a diamond shapedepitaxy).

In step 108, fabrication process 100 recess oxide and SiGe. In oneembodiment, fabrication process 100 recesses the oxide (deposited instep 106) and a portion of the SiGe epitaxy (of step 104), which exposesthe tops of the fins (formed in step 102). In an example embodiment,fabrication process 100 recesses the oxide, which exposes the top of theepitaxial growth on the fin (e.g., the tip of the diamond shaped epitaxyon top of the fin). Then, fabrication process 100 removes the exposedtop of the epitaxial growth on top of the fins (e.g., utilizingreactive-ion etching (RIE)), which exposes the top of the fins. Invarious embodiments, fabrication process 100 utilizes RIE or otherlithography techniques to remove oxide and SiGe (e.g., chlorine-basedRIE chemistry, Argon (Ar) milling, etc.).

In an example, fabrication process 100 recesses flowable oxide 401 andSiGe epitaxy 311 of FET 400, which results in FET 410 (depicted in FIG.4B). FET 410 includes recessed oxide 411, which fabrication process 100recessed to expose the top of SiGe epitaxy 311 (depicted in FET 400).Fabrication process 100 recesses flowable oxide 401 (of FET 400) down toform recessed oxide 411, which exposes the diamond tips of thediamond-shaped epitaxy of SiGe epitaxy 311 on fins 203. Then,fabrication process 100 utilizes RIE to recess the diamond tips of thediamond-shaped epitaxy of SiGe epitaxy 311, which exposes the tops offins 203 (e.g., un-doped Si fins). In additional embodiments,fabrication process 100 can utilize other techniques to expose the topsof fins 203.

In step 110, fabrication process 100 selectively etches the fins. In oneembodiment, fabrication process 100 selectively etches and removes thefins exposed in step 108 (e.g., the un-doped Si fins in the source anddrain region of the FET). In an example embodiment, fabrication process100 removes the fins utilizing a selective dry etching process (e.g.,RIE or another etching process that removes the fins but not the oxide).In another example embodiment, fabrication process 100 removes the finsutilizing a selective wet etching process. In other example embodiments,fabrication process 100 removes the fins utilizing other semiconductorfabrication processes that are capable of removing the fins (e.g.,etching straight downward) and not removing the oxide. In an additionalembodiment, fabrication process 100 etches the exposed areas of the finsin the source and drain region but does not etch the portions of thefins that are under the gates (formed in step 102).

In an example, fabrication process 100 selectively removes (e.g., viaRIE) fins 203 from FET 410, which results in FET 500 (depicted in FIG.5A). FET 500 includes removed fins 501, which are the empty areas (e.g.,trenches) of FET 500 that are the result of fabrication process 100removing the fins (e.g., fins 203 of FET 410). In various embodiments,fabrication process 100 removes fins 203 utilizing a downward etchingprocess (e.g., a dry etch, a wet etch, etc.).

In step 112, fabrication process 100 grows fins. In one embodiment,fabrication process 100 epitaxially regrows fins in the areas of the FETthat were etched in step 110 (e.g., the resulting trenches). In anotherembodiment, fabrication process 100 grows fins that are comprised of adifferent material (e.g., different than un-doped Si) than the removedfins (etched/removed in step 110). For example, fabrication process 100regrows fins that are comprised of high Ge content SiGe (e.g., 75%SiGe). In other examples, the regrown fins can be SiGe, Boron dopedSilicon, Boron doped SiGe, higher percentage SiGe, or other types ofmaterials.

In an example, fabrication process 100 grows (via selective epitaxy)regrown fins 511 in removed fins 501 (of FET 500), which results in FET510 (depicted in FIG. 5B). In one embodiment, regrown fins 511 are highGe content SiGe (e.g., 75% SiGe) fins. In another embodiment, regrownfins 511 are fins with a lower amount of boron doping compared to SiGeepitaxy 311. In additional embodiments, regrown fins 511 can becomprised of SiGe, Boron doped Silicon, Boron doped SiGe, higherpercentage SiGe, or other types of materials.

In step 114, fabrication process 100 grows SiGe on top of the fins. Inone embodiment, fabrication process 100 epitaxially grows SiGe (e.g.,highly doped SiGe (e.g., 35% SiGe)) on top of the regrown fins (fromstep 112). In an example embodiment, fabrication process 100 epitaxiallygrows SiGe tips (e.g., “tips” of the diamond) on a top exposed surfaceof the regrown fins to form a diamond-shaped unmerged epitaxy structurein the source and drain region of the FET (e.g., substantially similarto the diamond-shaped structure formed in step 104). In anotherembodiment, step 114 is optional and fabrication process 100 proceeds tostep 116 without performing step 114.

In an example, fabrication process 100 grows SiGe tips 601 (via epitaxy)on top of regrown fins 511 (of FET 510), which results in FET 600(depicted in FIG. 6A). SiGe tips 601 and SiGe epitaxy 311 combine toform a diamond-shaped unmerged epitaxy structure (e.g., the structureformed in step 104). In an example embodiment, SiGe tips 601 arecomprised of highly doped SiGe (e.g., 35% SiGe).

In step 116, fabrication process 100 recesses the oxide. In oneembodiment, fabrication process 100 recesses the oxide (deposited instep 106 and first recessed in step 108) down in the FET. In an example,fabrication process 100 recesses oxide in FET 600 (i.e., recessed oxide411 in FIG. 6A), which forms recessed oxide 611 in FET 610 (depicted inFIG. 6B). In various embodiments, fabrication process 100 utilizes RIEor other lithography techniques to remove and/or recess oxide. Inexample embodiments, FET 600 includes regrown fins 511 and the fin areaunder gate 214 (and gates 204) is an un-doped Si fin. In an examplewhere regrown fins are high percentage SiGe, the stressor of FET 600 canbe increased (in relation to using Si). Utilizing high percentage SiGecan also enhance strain in FET 600 and form “ultra-sharp” junction inFET 600. In another example embodiment, FET 600 can need no, or verylittle, additional diffusion or annealing. In an additional embodiment,fabrication process 100 can operate utilizing bulk Si and/or bulk FinFETmaterials.

What is claimed is:
 1. A field-effect transistor (FET) structurecomprising: a fin formed on a semiconductor substrate; a gate formed ona portion of the fin and the semiconductor substrate, wherein theportions of the fin under the gate is a different material than theportions of the fin that are not under the gate; epitaxially grownsemiconductor material on the fin; and an oxide covering at least aportion of the epitaxially grown semiconductor material and the fin. 2.The FET structure of claim 1: wherein the semiconductor material issilicon-germanium (SiGe); and wherein the epitaxial growth is adiamond-shaped unmerged epitaxy.
 3. The FET structure of claim 1:wherein the portions of the fin under the gate is un-doped silicon; andwherein the portions of the fin that are not under the gate aresilicon-germanium (SiGe).
 4. The FET structure of claim 1: wherein theportions of the fin under the gate is un-doped silicon; and wherein theportions of the fin that are not under the gate are boron doped silicon.5. The FET structure of claim 1: wherein the portions of the fin underthe gate is un-doped silicon; and wherein the portions of the fin thatare not under the gate are boron doped silicon-germanium (SiGe).
 6. TheFET structure of claim 1; wherein the portions of the fin under the setof gates is un-doped silicon; and wherein the portions of the fin thatare not under the gate are silicon-germanium (SiGe) that includes ahigher concentration of SiGe than the epitaxially grown semiconductormaterial.
 7. The FET structure of claim 1, wherein the oxide is aflowable oxide.